Our bodies suffer pain for a myriad of reasons. There may be systemic syndromes like Rheumatoid arthritis or Fibromyalgia. There may have been a trauma to a part of the body from an accident. Or, there may be pain from long term overworking of the muscles. High stress levels may have created high levels of tension in the musculature of parts of the body.
Muscles get into habituated patterns of holding or moving. This keeps them shortened, or tight, for long periods of time. Tight muscles start hurting when they are no longer able to continue to be tight. For instance, sitting at a computer can put your body into positions that create pain. Standing at the checkout counter scanning items can create repetitive movement patterns which, left unchecked, lead to chronic pain.
Very tight muscles often produce a phenomenon called referred pain. If these tight muscles are pressed at the correct spot, called a trigger point, pain or sensation will be felt someplace distant to the pressure. There are trigger points in the neck that, when pressed, will refer pain to the shoulder or down the arm. When the trigger point is massaged and released, the referred pain will disappear.